crossJoin 方法交叉连接指定数组或集合的值，返回所有可能排列的笛卡尔积：

$collection = collect([1, 2]);
$matrix = $collection->crossJoin(['a', 'b']);
$matrix->all();

/*
    [
        [1, 'a'],
        [1, 'b'],
        [2, 'a'],
        [2, 'b'],
    ]
*/
$collection = collect([1, 2]);
$matrix = $collection->crossJoin(['a', 'b'], ['I', 'II']);
$matrix->all();
/*
    [
        [1, 'a', 'I'],
        [1, 'a', 'II'],
        [1, 'b', 'I'],
        [1, 'b', 'II'],
        [2, 'a', 'I'],
        [2, 'a', 'II'],
        [2, 'b', 'I'],
        [2, 'b', 'II'],
    ]
*/